The claim is that this mitigation avoids putting fences everywhere, and it is not clear to me that the performance hit on an interpreter's own code would necess

The claim is that this mitigation avoids putting fences everywhere, and it is not clear to me that the performance hit on an interpreter's own code would necessarily be worse than for any other binary. I would guess the specific additional issue for interpreters is any sequence of instructions it creates dynamically (e.g. JIT compilation) and then has executed, and whether the interpreter provides the means (either intentionally or through a bug) for the attacker to influence that code generation such that it tests an address that the attacker should not have access to. I would guess that any program, whether interpreted or compiled, that is capable (again, either intentionally or through a bug) of generating instruction sequences and then having them executed, could be used to exploit Spectre. This would include any program capable of writing a binary file and then running it as a process or dynamically loading it as a library or native-code extension, if allowed to do so by the OS.